I would like to make a water cube however I would only one plane/surface to have the water simulation active
Ok! Sounds like fun.

Create your cube to set it to be a fluid simulation bound. Fill it with fluid using another cube and gravity should hold the side and bottom faces flat. It may need a few frames at the beginning to settle.

In theory, the simulation should bake and only the top surface has room to move. If there are any inconsistencies, use the boolean modifier to clean up the sides.

If you want the fluid to flow over the edge that is a whole nother step of complexity that I feel you haven't asked for. If this is what you want please update your question using the edit button.

If you are confused about any of the topics I have brought up and are not in italic, I will help you. Any in italics are easily found on the internet so please don't ask.

Not meaning to be rude but no need to repeat what is already freely available :D

Good luck with your project and I hope I helped,

BFB